10 hour Urinary Free Cortisol is basically the same as a 24 hour UFC (in fact, most labs run it as a 24 hour sample), but you collect your urine for 10 hours only, between 10pm and 8am.
This test is used specifically to look for a flipped diurnal rhythm...if your body produces too much cortisol at night, most often in patients who have mixed test results. This is the time of day your cortisol production should be at it's lowest, so it is a great time of day to test for excess cortisol (as we'll also discuss later with midnight testing).
The result used for this specific test is the cortisol/creatinine ratio. My endocrinologist uses a cut off of 16 and above (or is it just above?) for diagnosis. Sometimes the labs don't calculate it all (Some do routinely, some don't) and you have to specifically request the ratio.
A word to the wise about dealing with labs if they aren't used to this test: They can get confused if there is no "10 hour UFC" lab listed in their computers. I always tell mine to order it AS a 24 hour UFC when that is the case. My favorite local lab has no qualms asking me how to order testing correctly, which I MUCH prefer to wasting my time, pee, and money! (Isn't it nice to be treated like you know what you're doing?!)
The one key here is that THEY MUST ENTER THE TOTAL VOLUME AND COLLECTION HOURS IN THE COMPUTER! If not, you can call back later to fix it assuming they run the test and don't just toss the sample, but that's annoying to have to do too. And sometimes you run into people who act like you're the one who's confused or made the error. Just ask my friend who takes pictures of her jugs' labels...she's had to offer to send them a few times as proof!